Geotextile fabrics are commonly used to stabilize or reinforce earthen structures such as retaining walls, embankments, slopes and the like. Existing technologies include polyolefins (e.g., polypropylene and polyethylene) and polyesters which are formed into flexible, grid-like sheets. The sheets are stored on rolls and placed at the job site in one or more spaced apart generally horizontal layers depending on the height and reinforcement requirements of the earthen structure.
Despite ease of manufacture and installation, polyolefin and polyester grids are low modulus of elasticity materials typically having Young's moduli on the order of about 10,000 to about 75,000 psi for polyolefin grids and from about 75,000 to about 200,000 psi for polyester grids. Such low modulus products display high strain when subjected to the stresses in typical earthen structures. In some cases overlying soil and other forces associated with or imposed upon the earthen structure may induce as much as twelve inches of strain in polyolefin grids directions substantially transverse to the face of the earthen structure. Strains of this magnitude may destabilize not only the soil structure itself but also nearby structures such as buildings or roadways directly or indirectly supported by the soil structure.
Polyolefin grids may also undergo considerable creep when subjected to substantially constant loadings of the nature and magnitude of those typically exerted by or upon earthen structures. Thus, even if the short term strains are innocuous, the long term creep effects of polyolefin grids may be sufficient to threaten the integrity of the reinforced earthen structure and its surroundings.
Geotextile fabrics incorporating high modulus of elasticity materials have also been proposed for reinforcement of roadway structures. Examples include roadway reinforcement fabrics as described in U.S. Pat. Nos. 4,699,542, 4,957,930, 5,110,627, 5,246,306 and 5,393,559. These fabrics typically comprise elongate grid-like sheets wherein substantially parallel strands of high modulus material such as glass fiber rovings or the like extend in the longitudinal (or "warp" or "machine") direction of the fabric and in the transverse (or "weft" or "cross-machine") direction thereof. The glass strands are connected to one another so as to form an open grid and the entire assembly may be coated with a resinous material. Glass fiber roving strands have far higher moduli of elasticity and creep resistance than comparably sized polyolefin or polyester strands. For instance, the modulus of elasticity of a typical glass fiber strand in a geotextile fabric may be on the order of about 1,000,000 to about 4,000,000 psi. Glass strands can thus withstand much greater stress and undergo much less strain than comparably sized polyolefin or polyester strands. As such, glass-based geotextile fabrics generally provide superior reinforcement of earthen structures in relation to polyolefin or polyester grids.
The resinous coating material is applied to the glass fiber strands at a level of 10% to 15% DPU (dry-weight pick up), i.e., 10 to 15 parts dry weight of resin to 100 parts by weight of glass fiber. The resin coating is sufficient to protect the glass fiber strands from the comparatively benign installation and environmental conditions associated with roadway reinforcement applications. Additionally, the coating provides slight to moderate stiffness to the fabric such that it may be stored in rolls and easily handled at the job site.
In research and development culminating in the present invention, it has been observed that the commercial embodiments of the roadway reinforcement fabrics disclosed in the aforementioned U.S. patents, which are manufactured by Bayex Limited of Ontario, Canada, under the trademark GlasGrid.RTM. are unsuitable for soil reinforcement applications. More specifically, the resin which impregnates the fabric is incapable of withstanding the more rigorous physical and chemical demands associated with typical soil reinforcement applications. Most soil includes uncoated particles and stones which can be highly abrasive. In contrast, the aggregate used in asphaltic concrete is coated with asphalt which essentially eliminates the abrasiveness of the aggregate. Indeed, the art of reinforcing asphaltic concrete roadways remains somewhat underdeveloped and inexact. This may be due at least in part to the fact that roadway reinforcement materials do not experience the considerable exposure to potentially damaging factors that are routinely encountered by soil reinforcement materials during their installation and use.
In contrast to road reinforcements, use of reinforcements for soil stabilization is an established science. Longstanding and extensive reference texts and test procedures (e.g., ASTM, Drexel Test Procedures, FHWY Tests, etc.) have been developed that establish soil stabilization and the usage of reinforcements therein standard science. Upon examination of this field of technology and fabric reinforcement used therein, it was determined that rugged and rollable fiberglass fabric had not been successfully used in soil stabilization even though the science of soil reinforcement was well established.
Upon examination of existing soil reinforcements such as polyolefin and polyester grids, it was determined that design and usage of existing reinforcements required the grid structures to come under high strains to effectuate their soil reinforcement characteristics. Standard designs of wall structures or embankments using such grids require allowance for 5% to 10% strain levels on the grid structures in order to stress them sufficiently to capitalize upon their tensile reinforcement capabilities. With the use of fiberglass, however, which typically exhibits an ultimate strain of less than about 2%, it became apparent to the present inventor that such a reinforcement material may have applications in earthen structure designs that could tolerate only small strains. With this objective in mind, further investigations into a possible fiberglass soil reinforcement were conducted.
Standard engineering practice requires the consideration of a number of factors when selecting reinforcements for use in soil applications. Such factors typically include: (1) chemical resistance, i.e., the resistance of the reinforcement material to tensile degradation in various chemical environments, (2) UV resistance, i.e., the deterioration of a material's reinforcement properties responsive to ultra-violet (UV) radiation exposure, (3) construction damage resistance, i.e., the tensile strength retention capability of reinforcements under construction conditions using different soils (e.g., stone size distributions from fine silt to 3" coarse stone), (4) creep resistance, i.e., the property of a material to stretch and lose tensile strength with time while under stress. And, because coated fiberglass fabric materials were being considered by the present inventor, it was also necessary to consider the friction characteristics between reinforcements and surrounding soils which characteristics are dependant on the fabric's mesh opening size and coating chemistry. Although it had not been successfully produced, the present inventor believed that a reinforcement could be designed with fiberglass as the base reinforcement coated with a thermoplastic coating sufficient to satisfy these essential soil reinforcement design considerations.
Initial investigations for useful fiberglass soil reinforcement were focussed upon commercially available GlasGrid.RTM. asphaltic roadway reinforcement, a flexible coated fiberglass reinforcement available in roll form. It was quickly determined that the 10 to 15 DPU coating was insufficient to protect the fiberglass, which is a brittle material, from the harsh construction conditions associated with the erection of earthen structures. Testing indicated up to 70% tensile strength loss under construction situations was possible, thereby rendering GlasGrid.RTM. impractical as a soil reinforcement material. It is believed that the bitumen coated aggregates (standard particle size ranging from 1/16 to 1") used in asphalt represented a much less abrasive environment than that observed in soil applications which enabled GlasGrid.RTM. to be of beneficial use in asphaltic roadway installations but not soil structures. In light of this testing, the present inventor believed that a substantially different coating would have to be employed in order to render a GlasGrid.RTM.-type product useful as a reinforcement for earthen structures.
The standard GlasGrid.RTM. product that was tested had a grid opening size of 12 mm to 8 mm to allow for asphalt overlay adhesion to existing roads. This grid opening size was acceptable for the comparatively small aggregates used in asphalt roadway designs. In contrast, however, standard designs for soil reinforcement mesh openings are characteristically about 1" to as large as 12" to allow for proper aggregate interlock through the reinforcement. In addition to its unsuitable coating, the grid opening size of the standard GlasGrid.RTM. materials also contributed to the failure of GlasGrid.RTM. as a viable soil reinforcement product.
A fiberglass-based soil reinforcement fabric is described in "Walls Reinforced with Fiber Reinforced Geogrids in Japan" authored by K. Miyata and published in Vol., 3, No. 1, Geosynthetics International (1996). The design referred to therein is a fiberglass reinforcement with a rigid coating based on a vinyl ester resin (thermosetting rather than thermoplastic chemistry).
Fiberglass embedded in thermosetting resin has favorable creep characteristics, as well as good chemical resistance and abrasion resistance. However, the difficulty with this technology when deployed in soil reinforcement applications is that the thermosettings coatings render the material so stiff that it cannot be formed into rolls for rapid and convenient field application. Such products must be manufactured and sold as board-like sheets which would make them impractical for large scale soil reinforcement applications. From inception, the present inventor sought coated fiberglass reinforcement available in roll form to allow for easy unrolling in the field. Fiberglass reinforcement fabric embedded in thermosettable resins does not satisfy this criterion.
Use of vinyl ester resins also necessitates that measures be taken to assure their safe handling and disposal. Vinyl ester resins require dispersing solvents such as styrene for proper handling and processing. Solvents such as styrene are toxic, pollutant and have a low flash point (e.g., 88.degree. F. for styrene monomer). And, styrene and many other solvents suitable for dispersing vinyl ester resins have either been identified as or are suspected of being carcinogens. As such, precautions such as mandatory protective worker clothing and equipment, as well as extensive material handling training, must be implemented to prevent harm to the worker and the environment. Such measures add to the cost of manufacturing which, in turn, increases the cost of the vinyl ester resin impregnated geotextile end product.
An advantage exists, therefore, for a high modulus, open mesh, resin impregnated, geotextile fabric which is comparatively safe and inexpensive to manufacture, easy to handle and store in roll, sheet or other form, and is resistant to chemical degradation when used to reinforce earthen structures.